Deregulation of the expression of myc gene family members is required for tumor progression and maintenance, and is associated with many highly aggressive and poorly differentiated human cancers. In some tumors even transient downregulation of Myc results in tumor regression. Myc functions as part of a network of interacting transcription factors each possessing related but distinct domains involved in protein interaction and DNA binding. Through this basic-helix-loop-helix-zipper (bHLHZip) domain, Myc forms highly specific heterodimers with Max. Myc-Max heterodimers bind ~15% of genomic loci, often resulting in transactivation of genes involved in cellular growth, proliferation, metabolism, apoptosis and differentiation. Conversely, Mxd transcription factors competitively bind to Max and repress Myc gene targets, acting as Myc antagonists. Here we propose to employ a newly developed technology to screen a library of ~600,000 variants of the Max zipper domain and identify peptides that will specifically inhibit Myc-Max dimerization, but not Mxd-Max. Because Mxd-Max heterodimers repress genes activated by Myc-Max we predict that disruption of Myc-Max alone will result in inhibition of Myc driven cell proliferation. Peptide inhibitors derived frm our screen will be tested in biochemical assays and tumor cells. Using computational modeling, directed evolution and biological validation, we will assess the molecular and functional mechanisms of inhibition to ensure protein specificity and therapeutic efficacy in reducing levels of activated Myc and attenuating tumor growth